The relation of Body Mass Index to Mechanical Properties in Normal and Overweight Individuals

Abstract

Background: The study was to determine the relation of body mass index to muscular mechanical properties in normal and overweight individuals

Methods: A total of 172 participants (86 males and 86 females, mean age; 26.00±5.45 years) were participated. The participants were assigned in groups base on BMI classification (normal (BMI=18.50-24.99 kg/m²), overweight (BMI=25.00-29.99 kg/m²). The biceps brachii (BB), biceps femoris (BF) were measured bilaterally using the "MyotonPRO" device.

Results: Bilateral BB and BF stiffness, and BB elasticity were found significantly difference between normal and overweight group (p<0.05). Also the only left BB tone was found different (p<0.05) while other mechanical parameters found similar (p>0.05). In sex based BMI sub-categories comprasion; the bilaterally BB and BF stiffness and BF tone were found higher, and only the right BB elasticity was decreased in overweight male group (p<0.05). While the other mechanical parameters were found similar (p<0.05). The right BB stiffness and elasticity were found higher in overweight female group (p<0.05). No statistical difference was found in other parameters for female comparison (p>0.05).

A weak positive correlation was found between the right-left BB tone and stiffness and BMI. Also, a weak positive correlation was revealed between the right BB elasticity (p<0.05). No correlation was determined in other mechanical paramaters (p>0.05).

Conclusions: The bilateral BB and BF stiffness increased and BB elasticity decreased as BMI increased. The BB and BF mechanical properties were affected more in males than females when BMI considered.

Introduction

Obesity is a significant health problem that is gradually increasing. It can be defined as excessive fat accumulation in a way that can disrupt health, and it predisposes to chronic diseases (1). The cardiovascular and metabolic consequences of obesity have been studied extensively and less attention has been paid to investigation muscle function. The calculation of body mass index (BMI) is the simplest indicator of obesity (2,3). An increase in BMI may have direct or indirect effects on muscle function, balance, postur and physical activity (4,5).

Muscles and tendons have important viscoelastic properties. The force transfer, energy storage and recoil, spinal reflex activity need these features, so the proper movements and control of joints are achieved (6). The viscoelastic properties are not fully compatible with the spring model but can be explained by relatively simple elastic models (7). Among these features, stiffness and elasticity are most important for the performance and efficiency of human movements, and have an opposite relationship with each other. If a structure becomes too flexible, overloading may occur with the reduction of force. If it becomes too rigid, there may be an increase in forces in the kinetic chain (8). In both cases, it prepares a risk for musculoskeletal injuries.

While the body weight increased in obesity, more mechanical tension is put on soft tissue and joint structures. Downward vertical forces can affect these structures, such as osteoarthritis and plantar fasciitis mostly happened which linked with these forces. Other studies have focused an excess of fat infiltration into muscles to decreased muscle strength and power and to functional limitation. There were few studies that investigate the effects of BMI on the mechanical properties of muscles (9). The different methods used to evaluated these properties such as elastography, ultrasonography or force plates (10-14). In a study using a force plate, the gastrocnemius stiffness of non-obese healthy individuals and obese people were compared and it was stated that obese people had more stiffness (14). Also, the elastography was used to predict for BMI and mechanical properties relation, it was stated BMI had a weak relation with the upper trapezius stiffness (15). But the other studies with different technological assessment were not found relationship between mechanical properties and BMI (16,17). 

Accessing these systems is not always possible or may be limited in clinics because of high purchasing and maintenance costs and requirement of technical expertise (11). Thus, there is a need for objective, cost-effective, reliable, valid and easy-to-use methods to evaluate the mechanical properties of the musculoskeletal system (11). More recently, a new hand-held device known as MyotonPRO (Müomeetria Ltd., Tallinn, Estonia) was introduced. MyotonPRO offers a non-invasive, cost-effective and quantitative method for measurement of the mechanical properties of the musculoskeletal system (11). Objective measurement of soft tissue viscoelastic properties provided by MyotonPRO has high test-retest reliability and repeatability (11). The sternocleidomastoid (SCM) and upper trapezius (UT) muscle stiffness and elasticity were examined by myotonometric evaluation in adult females, and it was observed that there was a weak correlation between the UT elasticity and BMI and a moderate correlation between the SCM and UT muscle stiffness and BMI (18). Excessive body weight and its mechanical effect may prone to change viscoelasticity in extremities than cervical region when considering to vertical load. Further on, to authors knowledge upper and lower extremity muscles never related with obesity.

In the light of various literary data, it is not clear that BMI would be a factor in effecting the mechanical properties. The purpose of this study was to determine the relation of body mass index to muscular mechanical properties in normal and overweight individuals. It was hypothesized that increased BMI lead to higher stiffness and lower elasticity.

Materials And Methods

Participants

A total of 172 participants (mean age: 26.00±5.45 years) were participated. Sedentary participants who had proper range BMI (18-30 kg/m²) with physical activity levels of ≤ 300 MET min/week according to the international physical activity survey score were included in the study (19). Participants were excluded if they report; having systemic or metabolic disease, having psychological disease or drug use, having disease that might cause muscle atrophy, having musculoskeletal surgery in the last three months, having BMI more than 30 kg/m² or less than 18 kg/m².

The ethics committee approval numbered 2020/101 and dated 16.12.2020 was obtained from the non-invasive research ethics committee of Hasan Kalyoncu University, Faculty of Health Sciences. All participants voluntarily involved and they were informed about the content and purpose of the study and signed the consent form.

Procedures

The physical characteristics and demographic information of the participants were recorded prior to the test. Weight was evaluated using an electronic scale GSE 450 (GSE Scale Systems, Novi, Michigan), and height was evaluated using a standard stadiometer. BMI was calculated by dividing the weight in kilograms by the square of height in meters. Participants were divided into two subcategories according to sexes and BMI range: Normal (BMI=18.50-24.99 kg/m²) (n=86), Overweight (BMI=25.00-29.99 kg/m²) (n=86). 

It was stated that the partipicants should not consume alcohol for at least 24 hours and not engage in strenuous physical activity for at least 48 hours before the test (20). The tone and viscoelastic properties of the biceps brachii (BB) and biceps femoris (BF) muscles were evaluated bilaterally using a Myoton Pro (Müomeetria Ltd., Tallinn, Estonia) device. This device is known to have good to excellent reliability in healthy participants (21, 22). It can be used for objective diagnosis and monitoring in soft tissues in terms of validity and inter-user reliability (23, 24).  

The BB mechanical properties were evaluated by palpating the lateral end of the acromion and the cubital fossa in the middle from the ¾ of the distance between them with the individual in the resting supine position (25). Concerning the BF, the individual lay in the prone position and was asked to contract the hamstring muscle after placing a pillow under the ankle. The muscle was palpated while the individual was contracting it. Along with the contraction, the most prominent part of the muscle was marked and measured in muscle contraction, as suggested by Gavronski et al. (26). These muscles were preferred since they had been studied previously in many studies (27,28). For each measurement, mean deviation, median and 95% confidence interval were given, and mean values obtained from three consecutive measurements from the reference points were used in statistical analysis. 

The Myoton device provides data on three different properties. Tone (f) indicates a passive or resting muscle state without oscillation frequency (Hz), voluntary contraction (29). Stiffness (N/m) indicates resistance to any contraction or external intervention (26). Elasticity (D) is obtained as a logarithmic reduction of the natural oscillation of soft tissues. The increase in the number in the measurement obtained means the decrease in elasticity and is inversely proportional (29). The measurement creates a short-duration (15 ms), low-force (0.40 N) mechanical stimulation that induces damped natural oscillations of the tissues after the constant pre-stimulation (0.18 N) of the probe placed perpendicular to the muscle (3 mm in diameter) and is obtained by recording oscillations using an accelerometer (29).

Statistical Analysis

Descriptive statistics were presented as mean ± standard deviation. The Shapiro-Wilk test was used to check whether the data were normally distributed. The Mann-Whitney U test was used to compare differences between two groups (BMI; normal and overweigt, sex; males and females) for non-normally distributed data. The relationship between numerical variables was evaluated by Spearman correlation. As Spearman's rank correlation coefficient, 0.00-0.10 was interpreted as very weak correlation or no correlation, 0.10-0.39 as weak correlation, 0.40–0.69 as moderate correlation, 0.70–0.89 as high correlation, and 0.90–1.00 was interpreted as very strong correlation (30). 

Statistical analysis was conducted using Windows version 24.0 for SPSS (IBM Corp. Armonk, NY IBM Corp.), and the value p <0.05 was considered statistically significant. The minimum total number of participants required for each group was determined to be 44 (α=0.01) in order to determine the expectation that there would be a significant difference between three different BMI groups at the large effect level (f=0.75) obtained by referring to the published article with a power of 0.90. G-power program version 3.9.1.7 was used in power analysis (14). 

Results

A total of 172 healthy participants (mean age; 26.00±5.45 years, mean height; 1.69±0.08 m, mean weight;70.88±10.99 kg), including 86 females (mean age; 26.59±5.39 years) and 86 males (mean age; 25.41±5.47 years) were participated in this study. The 86 male participants; 43 (50%) were in normal group, 43 (50%) were in overweight group. The 86 female participants; 43 (50%) were in normal group, 43 (50%) were in overweight group.

Correlation between the BB and BF mechanical properties and BMI

All participants

A weak positive correlation was found between the right-left BB tone and stiffness and BMI (p<0.05). Also, a weak positive correlation was revealed between the right BB elasticity (p<0.05). No correlation was determined in other mechanical paramaters (p>0.05) (Table 1).

Females

A weak positive correlation was found between the right BB elasticity and BMI (p<0.05). No correlation was detected in the other mechanical properties of the BB and BF (p>0.05) (Table 1).

Males

A weak positive correlation was found between the right-left BB tone and stiffness and BMI (p<0.05). A weak positive correlation was revealed between the left BB stiffness, the right BB stiffness-elasticity and BMI (p<0.05). No correlation was determined in other mechanical paramaters (p>0.05) (Table 1).

Table 1: The Relationship Between the Mechanical Properties and BMI 

**p< 0.05,   *p< 0.05. r: Spearman’s rank correlation. Abbreviations:  BB: biceps brachii, BF: biceps femoris, Hz: Frequency, N/m: newton/meter,  log: logarithmic reduction.

Comparison of mechanical properties in BMI groups

All participants

A statistical difference was found bilaterally in stiffness of BB and BF, and BB elasticity (p<0.05). While the right and left BF elasticity was found similar (p>0.05). Only the left BB tone of overweighted group was found to be higher compared to normal group (p<0.05). While the other muscle tones was found similar (p>0.05) (Table 2).

Males

The statistical differences were found bilaterally in stiffness of BB and BF muscles. Only the right BB elasticity of overweighted group was found to be higher compared to normal group (p<0.05). While the other muscle elasticities were found similar (p<0.05). The left and right BF tone was higher in overweighted group (p<0.05), while the right and left BB tone was found similar (p>0.05) (Table 2).

Table 2. The comparison of mechanical properties in BMI groups 

* p <0.05. Abbreviations:  T: Total, M: Male, F: Female, BB: biceps brachii, BF: biceps femoris, Hz: Frequency, N/m: newton/meter,  log: logarithmic reduction

Females

There was found significant differences in the right BB stiffness and elasticity (p<0.05). No statistical difference was found in other parameters (p>0.05) (Table 2).

Discussion

This study was conducted to determine the relation of body mass index to muscular mechanical properties in normal and overwieghted individuals. There was found a weak relation between BMI and the mechanical properties of the BB and BF muscles. The bilateral BB and BF stiffness increased as BMI increased, and the bilateral BB elasticity decreased. The overweighted males showed increased bilateral BB, BF stiffness, and BF tone. Only the right BB stiffness and elasticity were found higher in overweighted females. The BB and BF mechanical properties were affected more in males than females.

Resting muscle tone is classified into two categories as neural and non-neural. If there is no neural activation, muscle tone contains passive stiffness and viscoelastic properties (25). When all participants were examined, a weak positive correlation was observed between BMI and the bilateral BF tone and stiffness in all participant and males. Also, the right BB stiffness and elasticity were positively correlated in males. While only the right BB elasticity was found to be weakly positively correlated in females. The tone and stiffness relation between BMI in the lower extremity suggests that it can be a different neural and muscular adaptations that people with mechanical load. In a study conducted with 12 people with obesity (BMI>27) adolescent girls and 12 healthy girls, it was reported that with increased mechanical load in people with obesity, adaptation would occur in muscles and nerves, and as a result, people with obesity might have a larger pennation angle, anatomical cross-sectional area and muscle thickness (31). While this advantage in mechanical loading is observed in the positive direction in the lower extremity depending on the increase in weight, it may explain that it is in the negative direction in the upper extremity. The upper extremity, which lacks mechanical loading, and the sedantary life, may bring along a disadvantage that will result in the loss of the cross-sectional area and contractile components. In studies comparing athletes and sedentary participants, it is stated that sedentary participants have a smaller cross-sectional area (32). This opposing relationship proves that muscular and neural structures will develop different adaptations in the upper and lower extremities. 

There was found a positive weak relation between the right-left BF tone and stiffness with BMI base on all participants and males comparison. Also the right and left BB stiffness had a weak relation with BMI in males. Kocur et al. evaluated the relationship between the SCM muscle stiffness and elasticity and BMI, it was indicated to be highly correlated with elasticity and moderately correlated with stiffness (33). Therefore, it was reported that males with high BMI had lower biceps brachii elasticity than females (14). In our findings, only the right BB elasticity had weak relation with BMI in all comprasion (BMI and sex based). Furthermore, no correlation was observed in other paramaters in females. Fat infiltration into skeletal muscles in people with obesity can create higher muscle stiffness and reduce flexibility compared to the people with non-obesity group due to the limitation of range of motion and stable posture (14). Moreover, the increase in adipokines, which regulate the production of metalloproteinases, prostanoids, and cytokines in adipose tissue, can affect stiffness and flexibility in overweighted people (34). The different elasticity relationship in the lower and upper extremities suggests that it may be caused by changes in adipose tissue according to sex.

When all participants were compared in the sub-groups according to BMI, bilaterally BB and BF stiffness were higher in overweighted people. Therefore, the right-left BB elasticity of overweight were higher compared to normal participants. Comparisons in males were close to the characteristics we obtained from all the participants above, while in females, mechanical properties were not affected, except for the right BB stiffness and elasticity. Along with excessive weight gain, adipocyte hypertrophy, intramuscular adipose tissue infiltration, an increase in fibrous components (a decrease in contractile elements), a decrease in the size and number of muscle fibers can be said to be the causes of decreased elasticity and increased stiffness (35-37). However, at this point, we think that adaptations that develop in daily life according to the mechanical loads on the lower and upper extremities will be the primary cause. Increased BMI can affect stability and may provide a biomechanical advantage by increasing the stiffness and tone of the lower extremity muscles due to excessive trunk oscillations in stance or walking. 

An inverse correlation between muscle tone and subcutaneous fat was previously observed in sedentary participants (25). Therefore, it can be assumed that more thickness of subcutaneous fat may alter the response of muscles, reduce their oscillation and frequency, and thus affect tone. In a study comparing female athletes with sedentary females, it was stated that athletes had low BMI values, which was the reason for the decrease in the percentage of subcutaneous fat and the high muscle tone (32). The calculation of BMI using height and weight in our study may have limited our study in terms of not measuring subcutaneous fat tissue thickness. At this point, we think that regional fat deposition together with BMI may be important for future studies. 

From a practical point of view, the increased tone and muscle stiffness in relation to BMI may lead to a decrease in the risk of falls, injury and overall muscular performance, resulting in a limitation in the ability to perform daily activities (38). In this study, increased BMI changes the mechanical properties of the muscles. The decrease in the muscular performance of people with obesity may indicate why physical activity is reduced or vice versa. 

Limitations

Our study provides important information for influencing BMI on the musculoskeletal system. But we had some limitations. Firstly, we used the classical BMI calculation using only height and weight, and we did not evaluate the adipose tissue thickness of the participants. We could have obtained clearer findings with the thought that there might be participants with different body compositions. If we could have evaluated whether the muscles were relaxed sufficiently in the resting position by an objective method such as EMG, it could help us better understand the mechanical properties, especially the tone. As it is, the findings of our study will help further investigate the relationship between obesity and the mechanical properties of the musculoskeletal system. 

Conclusion

A weak correlation was found between BMI and the mechanical properties of the BB and BF muscles. The bilateral BB and BF stiffness increased as BMI increased, and the bilateral BB elasticity decreased. The overweighted males showed increased bilateral BB and BF stiffness  and BF tone. Only the right BB stiffness and elasticity were found higher in overweighted females. The BB and BF mechanical properties were affected more in males than females. In addition to excess weight, increased stiffness and decreased elasticity may have adverse effects on other systems, especially physical activity, ambulation, and musculoskeletal system.

Declarations

Acknowledgements

The authors would like to thank all participants in this study.

Declarations

Ethical approval

This study was approved by the Ethics Committee for Non-Invasive Research Studies of Hasan Kalynocu University Faculty of Health Sciences (approval number 2020/101).

Informed consent

Informed writing consent was obtained from each participants before starting study

Conflict of interest

All authors declare that they have no conflict of interest.

Funding

No funded

Data Availability

Not applicable.

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